An alternator is a power generator that generates alternating current (AC) power through an electromagnetic induction occurred between an armature and a stator. The alternator usually applies to a car and is started by an internal combustion engine via belts and belt wheels, so as to supply electric power for electronic devices in the car. When the car is started, the alternator and a motor of the car are started by using the residual electric power from a car battery. When the motor is started, it triggers the alternator to operate and then the alternator begins to charge the car battery. Typically, the alternator includes a rotor assembly, a stator assembly, a rectifier, and a field diode.
The rotor assembly is rotatable with respect to the stator assembly, and includes a rotor core, a rotor winding, a rotor shaft, and a bearing. When an electric current flows through the rotor winding, the rotor winding is magnetized due to an electromagnetic induction to produce a magnetic field thereby. When the rotor assembly rotates, the magnetic field direction is changed accordingly.
The stator assembly encompasses the rotor assembly and consists of a stator core and one or more sets of stator windings. According to electromagnetic induction, the stator windings physically offset so that the rotating magnetic field produces currents.
The rectifier is electrically connected to the stator windings for converting the alternating current generated by the stator windings into direct current, and the direct current is used to charge batteries.
The direct current converted from the alternating current and generated by the stator windings is outputted by the field diode. Therefore, the alternator with the above structure can supply current to the stator windings by itself. This process is referred to as self excitation.
When the rotor assembly is turned on from a resting state, most of the alternators have to use the residual magnetism in the rotor core to build up a magnetic field, so that the alternator can generate sufficient power to produce the self excitation state thereof. The lowest rotating speed of the rotor assembly for producing the self excitation state of the alternator is referred to as the turn-on speed. Before reaching the turn-on speed, the alternator has to use the electric energy supplied from the battery to drive the rotor assembly to rotate, so as to perform the turn-on procedure.
Some large vehicles, such as agricultural vehicles and trolley trucks, are usually designed for specific purposes and are therefore not put into use so frequently. As it is known, a car battery is subject to self-discharge and the electric energy stored therein will gradually decrease. When the large vehicles mentioned above have not been started over a long time, the car batteries thereof tend to have insufficient battery power for driving the rotor assembly of the alternator to the required turn-on speed, making it difficult or unable to start the large vehicles.
On the other hand, some small vehicles, such as private cars and commercial vehicles for commuting or for other business purposes, are frequently started and accordingly, less suffer from the problem of insufficient battery to start the vehicles. However, the alternators for these cars and vehicles usually require a relatively high turn-on speed, which would consume more battery power.
In the alternator, the rotor core thereof is mainly used to transfer magnetic flux between the rotor assembly and the stator assembly to form a magnetic circuit, and must have the property of a permanent magnet to provide residual magnetism for generating electric power until the alternator reaches the state of self-excitation to keep the rotor assembly rotate continuously. With higher residual magnetism, the rotor can have lower turn-on speed.
The rotor core is usually made of carbon steel. The carbon steel can be categorized according to the carbon content thereof into low carbon steel, medium carbon steel and high carbon steel. The low carbon steel usually has relatively good ability of magnetic flux transfer and therefore increases the output of the alternator, such as output current or output voltage. However, the low carbon steel disadvantageously has relatively low residual magnetism when the external magnetic field is removed after magnetization. On the other hand, the high carbon steel has characteristics very close to those of the permanent magnet and has residual magnetism higher than the low carbon steel, but the high carbon steel has relatively low magnetic permeability. Thus, the prior art rotor core for an alternator is usually made of the medium carbon steel in order to maintain proper turn-on speed and output characteristics.
However, the rotor core made of the medium carbon steel has relatively high magnetomotive force drop (MMF drop) across the cross-section area of the rotor core. That is, the magnetic permeability is lowered to result in inferior output characteristics of the alternator. Moreover, the rotor core made of the medium carbon steel has a relatively low magnetic saturation flux density and, accordingly, relatively high weight, volume and mass moment of inertia, which could not be reduced further.
It is therefore desirable to develop an alternator that has reduced turn-on speed, high-output characteristics, and small volume without occupying too much space.